Personal cleansing with mild surface-active cleansing preparations has become a focus of interest. Some of the problems associated with mild bars comprised of synthetic detergents (synbars) are bar processability, firmness, lather and rinsing. The problems of formulating synbars are not limited to the performance characteristics of the finished bar. Most synbars which are made with certain mild surfactants are very difficult to fabricate. In contrast, the fabrication of "soap" bars is a well-worked-out engineering procedure involving milling, plodding and molding. Soap becomes quite plastic when warmed and can be easily plodded and molded under relatively low pressures. Most synthetic detergents and detergent-filler combinations do not become plastic and the machinery for fabrication must be specially designed. See U.S. Pat. No. 2,678,921, J. A. V. Turck, Jr., issued May 18, 1954. Ideal synbar processing should be fast and problem free in terms of milling, plodding and molding toilet bar formation. Most synbar processing falls short in this respect.
The development of soap-synthetic bars (synbars) dates back to World War II. U.S. Pat. No. 2,432,169, Hoyt, describes a bar having soap and alkyl benzene sulfonate as the active and a substantial proportion of corn starch as a binder; and U.S. Pat. No. 2,988,511, Mills and Korpi, issued Jun 13, 1961, describes a nonsmearing bar comprising alkyl glyceryl ether sulfonate, soap and binder. U.S. Pat. No. 2,982,735, Blinka and Grounds, Jr, issued May 2, 1961, discloses a detergent milled bar comprising soap, anionic detergent and starch. U.S. Pat. No. 2,987,484, Lundberg and Blinka, issued Jun 6, 1961, discloses a closed die injection molded detergent bar comprising alkyl glyceryl ether sulfonate and acyl isethionate. Some other uses and procedures for making alkyl glyceryl ether sulfonate are disclosed in U.S. Pat. Nos.: 2,094,489, Hulter, issued Sep. 28, 1937; 2,427,576, Smith, issued Sep. 16, 1947; 2,427,577, Smith, issued Sep. 16, 1947; 2,989,547, Whyte, issued Jun 20, 1961; 2,999,068, Pilcher et al., issued Sep. 5, 1961; and 3,024,273, Whyte et al., issued Mar. 6, 1962, all of said patents incorporated herein by reference.
It is noted that surfactant mildness can be measured by a skin barrier destruction test which is used to assess the irritancy potential of surfactants. In this test the milder the surfactant, the less the skin barrier is destroyed. Skin barrier destruction is measured by the relative amount of radio-labeled water (.sup.3 H-H.sub.2 O) which passes from the test solution through the skin epidermis into the physiological buffer contained in the diffusate chamber. This test is described by T. J. Franz in the J. Invest. Dermatol., 1975, 64, pp. 190-195; in U.S. Pat. No. 4,673,525, Small et al., issued Jun 16, 1987, and in copending U.S. patent application Ser. No. 294,832, Small et al., filed Jan. 9, 1989.
U.S. Pat. No. 2,894,912, Geitz, issued Jul 14, 1959, for "Isethionate Detergent Bar," discloses a detergent bar consisting essentially of from 30-70% of water-soluble alkali metal detergent salts of esters of isethionic acid with mixed aliphatic fatty acids having from 6 to 18 carbon atoms and an iodine value of less than 20, of which mixed acids at least 75% have from 12 to 18 carbon atoms and up to 25% have from 6 to 10 carbon atoms, from 2-10% of at least one water-soluble suds-boosting detergent salt selected from the group consisting of alkali metal and organic amine higher aliphatic fatty alcohol sulfates, alkyl aryl sulfonates, and higher aliphatic fatty acid taurides, from about 1% to about 9% water, from about 2.5% to about 25% of water-soluble higher fatty acid soap, and from 10-40% of at least one higher fatty acid having from about 12 to about 25 carbon atoms as a binder and plasticizer, said bar having a pH within the range from 6 to 8, measured as a 10% aqueous solution of the bar composition at 35.degree. C.
U.S. Pat. No. 4,180,470, Tokosh et al., issued Dec. 25, 1979, discloses a method for making improved 30-70% acyl isethionate detergent bars with from 2-6% of sodium alkoxy hydroxy propane sulfonate (a synonym for alkyl glyceryl ether sulfonate) with alkyl chains of from 8 to 22 carbon atoms in conjunction with a small amount of sodium chloride. C.sub.18 alkyl glyceryl ether sulfonate at 5% is used in an example. The added alkyl glyceryl ether sulfonate and salt are used to improve bar wear rate without adversely affecting its lathering characteristics.
U.S. Pat. No. 4,234,464, Morshauser, issued Nov. 18, 1980, for "Detergent Bar Composition and Binder Therefor," discloses a detergent bar in Example 6 which comprises: 45% sodium cocoyl isethionate, 5% alkyl amide, 37.5% stearic acid, 5.0% hydrogenated tallow glycerides, and 1% Polymer JR. Morshauser teaches that his detergent bars can contain up to 5% soap "without substantial detriment."
U.S. Pat. No. 4,012,341, Orshitzer et al., issued Mar. 15, 1977, for a "Unique All Synthetic Detergent Shampoo Bar," discloses a bar comprising a mixture of anionic and nonionic detergents. The Examples are primarily based on sodium lauryl sulfate, which is an unacceptable primary surfactant for the present invention.
U.S. Pat. No. 3,761,418, Parran, Jr., issued Sep. 25, 1973, for "Detergent Compositions Containing Particle Deposition Enhancing Agents," discloses detergent compositions including a bar, which main surfactant is alkyl sulfate, which is unacceptable for the mild skin cleanser of the present invention.
Major drawbacks of most synthetic surfactant toilet bar formulations are poor processability due to stickiness, harshness, poor lather, or poor rinse feel. The use of high sudsing anionic surfactants can yield acceptable lather volume. Unfortunately, the highest sudsing anionic surfactants are, in fact, poor in processabilty. While the prior art mild blends of sodium coconut/tallow alkyl glyceryl ether sulfonate are relatively good in lather potential, they are not so easy to process because of their stickiness or hygroscopicity. It will be appreciated that processability, mildness, lather, and rinsability make surfactant selection for mild synbars a delicate balancing act. Thus, it will be appreciated that rather stringent requirements for formulating mild synbars limit the choice of surfactants, and final formulations represent some degree of compromise. Mildness is often obtained at the expense of processability, effective cleansing, lathering, or rinsing, or vice versa. Needless to say, a superior processable synbar formulation with good mildness, good lather potential and good tinsability is hard to formulate.